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Humidity: A Mysterious Contaminant – How to Filter It and Why

Some consider compressed air filters as nothing more than dirt removal devices. Although dirt removal is an extremely important function of the filter, the truth is that water in both liquid and vapor forms is sometimes the primary target contaminant for removal. This reality is often overlooked and rarely understood because water in compressed air systems behaves in unique ways that can make it appear or disappear depending on conditions of both the atmosphere and the specific compressor system.

Donaldson offers products specifically designed to remove water in both liquid and vapor phases, as well as to properly treat and safely discharge the collected water.

DF Filter Housing
Step 1: Water Removal
  • Expansion: One way of drying compressed air is to expand it. In order for this method to work, there needs to be a Donaldson coalescing filter in front of the pressure regulator. The higher pressure air upstream of the regulator will have bulk water and aerosolized water entrained in the airstream. If this condensate pushes through the regulator, it will readily evaporate and re-humidify the air. Depending on the application, it is also best practice to put a particulate filter downstream of the regulator — a mechanical device that can inadvertently generate particles that may compromise a process or piece of equipment.

  • Refrigeration: Another method of water removal is to cool the air by using a refrigerated air dryer. If the temperature of the air is decreased, the maximum water vapor content at the cooler temperature will be lower, and condensation will form. It is important to protect the dryer from condensed water or oil emulsions by using a Donaldson coalescing filter. If the oil aerosols were allowed to enter the dryer, the evaporator heat exchanger tubing would quickly become coated and lose its flow and cooling abilities. Since the dryer will produce condensation through cooling, there needs to be a cyclone separator at the dryer outlet.

  • Adsorption: For the driest compressed air possible, desiccants are used to adsorb water vapor in the airstream. Donaldson point of use desiccant dryers come complete with all pre- and post-filtration required to optimize system performance and assure downstream air quality.
UFM-D Zero Air-Loss Drain
Step 2: Draining Water

All of the condensate produced by air drying needs to be dealt with properly because it is a dirty mixture containing water, compressor oil, dirt, and rust. It is not acceptable in most cases to use a solenoid valve or manual drain valve because of constantly changing atmospheric conditions, production loads, and compressor conditions. These are antiquated methods that are very susceptible to mis-adjustment leading to extreme energy waste or flooding of the filter. In a Donaldson DF filter housing, the condensate is automatically dispensed by way of an internal float drain. The float drain's operation can be tested manually during operation by pushing on the brass nipple at the bottom of the drain. For longer life and higher energy savings, the Donaldson UFM-D electronically controlled zero air-loss drain is employed. This is an easily serviceable external drain that uses a capacitive liquid level sensor and solenoid activated membrane valve to economically control condensate.

UFS / DS Series Oil/Water Separator
Step 3: Water Disposal

The condensate needs to be treated before it can be lawfully disposed of. This is accomplished by running all condensate drains to a Donaldson UFS / DS Series Oil/Water Separator. The UFS / DS takes the blow-down from condensate drains and separates the emulsion into clean water and waste oil streams. As condensate enters the UFS / DS, it removes particles by separating bulk material. The cleanest of the water (near the bottom of the separator) is drawn through an activated carbon container to remove trace dissolved hydrocarbons, and then discharged as clean wastewater.

Contact us for solutions to all the water and humidity issues discussed in this article.

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